The present invention is generally directed to regenerators for use in regenerative gas cycle cryocoolers and refrigerators. More particularly, the present invention is directed to regenerator materials used in connection with Stirling cycle cryocoolers.
Regenerative gas cycle cryocoolers, and in particular, Stirling cycle cryocoolers have increasingly been employed to achieve low cryogenic temperatures for a variety of applications in different fields. One particular application for Stirling cycle cryocoolers relates to the cooling of high temperature superconducting (HTS) materials. HTS materials are used in a number of applications including, for example, in front-end filters used in wireless telecommunications.
Stirling cycle cryocoolers offer a number advantages over other types of cryocoolers. First, the Stirling cycle is a thermodynamically efficient cycle that requires a relatively small amount of power to achieve cryogenic temperatures. Stirling cycle cryocoolers can also be manufactured into a relatively compact size. Moreover, the necessary components needed for the Stirling cycle can be economically incorporated into a cryocooler. Finally, Stirling cycle cryocoolers provide a long operating life that exceeds that of other more conventional cryocoolers.
Stirling cycle cryocoolers operate in a closed thermodynamic cycle in which a working gas such as helium is alternatively compressed and expanded using one or more pistons contained within a cylinder. In one design, Stirling cycle cryocoolers may contain two moving pistons (an expansion piston and a compression piston) as well as a stationary regenerator material. In another design, the so-called free piston Stirling cycle cryocooler, uses a single cylindrical expansion piston that contains the regenerator material, herein referred to as a xe2x80x9cmoving displacerxe2x80x9d. In either design, however, the regenerator material acts as a heat exchanger for the working fluid. In one portion of the Stirling cycle, the regenerator absorbs heat from the working fluid (i.e., helium gas) while in another portion of the Stirling cycle, the regenerator releases heat to the working fluid.
For high performance, the regenerator material should be thermally insulating in the axial direction (i.e., in the direction of the thermal gradient) while at the same time be able to exchange heat rapidly with the working fluid. Several types of regenerators have been used in the past in conjunction with Stirling cycle cryocoolers including beds of packed spheres, stacked layers of fine gauge metal wire, metal foils, steel wool, steel felt, and parallel plates with flow passages. It is also known that polyester such as pillow batting can be used as the regenerator material. With respect to the use of polyester as the regenerator material, it is known to pack the polyester into a moving displacer within a free piston Stirling cycle cryocooler using a series of xe2x80x9cwadsxe2x80x9d or xe2x80x9cballsxe2x80x9d of polyester.
When wads or balls of polyester are used, the displacer must be manually packed with the appropriate number of wads or balls for each cryocooler. This process, however, is extremely labor intensive and adversely impacts the yield and performance of the completed cryocoolers due to variations in the packing uniformity from cryocooler to cryocooler.
In some cryocoolers, the wads or balls are packed tightly while in others the wads or balls are packed loosely along the length inside the displacer. This packing problem, produces varying heat acceptor temperatures (i.e., cold end temperatures) for a given power input. Similarly, if a constant cold end temperature is desired, different cryocoolers with differing packing characteristics will have different power input demands. A need exists for a regenerator material that will produce constant or near constant cold end temperatures in differing cryocoolers that are powered by the same input power.
Another problem with using wads or balls of polyester is that there are interstitial spaces or gaps between the wads/balls that allow the working fluid to easily pass through without any heat exchange. This is particularly troublesome at the interface between the regenerator material and the inner diameter of the displacer where working fluid may readily pass along in the axial direction with little or no interaction with the regenerator material. These spaces or gaps can also create unwanted variations in cryocooler performance. There is thus a need for a regenerator material that will prevent these adverse variations in performance between different cryocoolers caused by gaps or spaces located within the regenerator material.
Finally, packing the wads or balls of polyester within the displacer of a Stirling cycle cryocooler requires considerable force to achieve the optimum packing density of polyester. The packing of the wads/balls, however, has traditionally been accomplished by hand (i.e., during manufacturing of the cryocooler, a person manually stuffs the displacer with the wads/balls of polyester) into the displacer and requires persons of considerable strength to force all of the polyester wads/balls into the displacer. Some people working on the manufacturing line simply do not have the strength to pack the polyester wads/balls into the displacer in a timely fashion if at all. Accordingly, there is a need for a regenerator material that can be loaded by any person in the manufacturing linexe2x80x94not just those individuals with extraordinary strength.
In a preferred embodiment of the invention, a regenerator material for use inside the moving displacer of a Stirling cycle cryocooler includes a plurality of circular disks formed from a synthetic felt. The plurality of circular disks form a stack within the displacer of the Stirling cycle cryocooler. The plurality of circular disks have outer diameters that are greater than the inner diameter of the displacer. It is preferable that the synthetic felt be made of polyester but other materials may be used such as, for example, polytetrafluoroethylene, polyimide, and polyamide.
In another aspect of the invention a method of filling a displacer with a regenerator material comprises the steps of forming a synthetic felt, punching out a plurality of disks out of the felt, wherein each of the plurality of felt disks have an outer diameter that is greater than the inner diameter of the displacer, loading the plurality of felt disks into the interior of the displacer, and closing the displacer.
It is an object of the invention to provide a regenerator material that can be used inside a displacer of a Stirling cycle cryocooler. It is a further object of the invention to provide a regenerator material that can be easily packed within the inside of a displacer. It is another object of the invention to provide a regenerator material that minimizes performance variations between different packed cryocoolers. It is yet another object of the invention to provide a regenerator material that prevents the unobstructed flow of working fluid through the regenerator in the axial direction. Other objects of the invention are described in detail below.